1. Field of the Invention
This invention relates to improvements to sailboard booms and particularly to improvements to clews that unit boom arms while providing an attachment point for the trailing edge of a sail.
2. Description of Related Art
Sailboard booms typically comprise a generally ovate structure having a mast end and a clew end interconnected by a pair of boom arms. The mast end attaches to the mast of the sailboard using a fitting tied to or made an integral part of the boom. The arms tie together at the clew end opposite the mast using a clew that serves as an attachment point for the trailing edge of the sail.
Clews are manufactured in a variety of configurations having in common a substantially U-shaped body comprising two studs adapted to connect to the clew ends of the boom arms, and a central portion carrying an attachment means for attaching the sail to the clew. Clews have been made of wood, tubular or solid metal, rubber, plastic and fiberglass. Clew geometry and material makeup has been the subject of experimentation to investigate strength and weight characteristics in keeping with the interest in lighter weight components for sailboard booms. Of recent interest has been the use of carbon fiber composite because of its long run fatigue resistance and superior tensile strength.
During operation, booms and boom components such as clews undergo rapid moment force cycling induced by wind dynamics transferred to the boom from the sail, by wave chop transferred through the mast and through the sailor's body, and by the pumping action used by sailors, particularly in top professional racing, to recover speed after a turn and to precisely trim the sail for maximum speed. Carbon fiber composite provides the long run best performing material, making it desirable material for the construction of clews. But it presents some unique construction problems.
Carbon fiber composite provides high tensile strength, but a clew molded from carbon composite can have no sharp corners or edges between its constituent parts. If carbon fibers must pass around a sharp corner or tolerate a crease transverse to their grain, the composite will not transfer load across the discontinuity created, thus providing almost no strength in that direction. Further, carbon composite cannot easily be welded or assembled from separate pieces. Providing isotropic strength characteristics in an object requires that layers of the carbon fiber be oriented in many directions to provide some moment strength about important axes of the object.
A carbon fiber composite clew having the conventional U-shape would experience moment and shear forces in three dimensions due to the fulcrum role it performs while holding one end of the boom arm which itself experiences significant moment forces. Because of the nearly 180 degree bend of the central portion of a typical U-shaped clew, it is impractical to construct it from carbon composite. The fibers must be carefully aligned to avoid bunching inside the curve and degrading directional strength reliability. For most industrial applications, this is impractical. A clew may be overbuilt to overcome these problems, but only at the cost of additional weight. Typically, enough weight must be added to the U-shaped clew to more than overcome the advantages of using the high strength, light weight carbon fiber composite. A need therefore exists for improvements to clew geometry so that it may be dependably constructed from lighter weight carbon composite material.
When manufactured, booms having varying degrees of flexibilty. Since stiffer brooms provide greater control, particularly with top professional racing, this flexing is undesirable. Flex in a clew adds to boom flex, especially due to the U-shape. A need exists for a clew that has minimum flex as a means to promote stiffness in the boom and provide greater board control to the sailor.
Clews often have adjustable tubes cooperating with or made a part of their arm studs to provide a plurality of initial tensioning positions relative to the trailing edge of the sail. In this fashion, a boom may be used with sails having various widths. A sail trim adjustment is usually included as part of the sail attachment means to fine tune the sail tension once an initial tensioning position has been selected. This usually amounts to a rope or lanyard tying the sail to the clew, but various pully and sheave devices have been devised to permit quick adjustments even during sailing. Because carbon composite has poor shear strength, however, clews made of the composite cannot readily include such conveniences. A need therefore exits for a quick sail trim adjustment means for clews made of carbon composite.